The present invention relates to a method and apparatus for ascertaining the positions of selected portions of elongated webs of flexible material, especially to a method and apparatus for ascertaining the positions of successive frames of one or more exposed and developed photographic films preparatory to introduction of such films into a copying machine. Still more particularly, the invention relates to improvements in a method and apparatus for automatic detection of positions of film frames for the purpose of identifying such frames by suitable indicia (e.g., notches) prior to introduction into a copying machine or the like.
German Pat. No. 1,285,317 discloses a method and apparatus for determining the positions of film frames in successive exposed and developed photographic films. The patent proposes to form a so-called holding function which is obtained by continuous scanning of light transmissivity of the film. The maximum transmissivity is assumed to be in the region of a frame line (it being assumed that each monitoring step includes the evaluation of transmissivity within a film portion whose length equals or approximates the length of a film frame). An edge portion of the frame line (such edge portion also constitutes the edge portion of the adjacent film frame) can be assumed to constitute an abrupt transition between areas of higher and lower light transmissivity. As a rule, one expects that the change of transmissivity at the opposite sides of an edge of the frame line will amount to at least three to five percent which should be sufficient to reliably detect such edge by resorting to photosensitive means. Thus, one expects that, when a film frame advances beyond the monitoring device, the latter detects a reduction or increase of transparency which is in the order of at least three to five percent, and such change of transparency is indicative of detection of a film frame.
The just outline procedure allows for reliable detection of film frames whenever the transition from a frame line to the adjacent film frame or vice versa is sufficiently pronounced, i.e., when the transmissivity of the frame line is appreciably different from the transmissivity of adjacent portion of the film frame. Problems arise when such procedure is resorted to for the detection of overexposed film frames because the frame lines which flank an overexposed frame are often exposed to light so that the transmissivity gradient is low or non-existent. The same applies for underexposed frames, especially film frames which were exposed to artificial light; in such films, the difference between the light transmissivity of front or rear portions of film frames and the light transmissivity of frame lines is practically nil.
German Pat. No. 1,797,439 discloses an apparatus with two scanning slots. The distance between the two slots, as considered in the direction of film transport, equals or approximates the length of a film frame. The light beams which pass through the two slots are transmitted to discrete photosensitive devices which operate independently of each other. Thus, if one of the photosensitive devices detects a sharply defined edge between a film frame and the neighboring frame line, such edge will be used as a reference point for determination of the other edge regardless of whether or not the other edge has been detected with the same degree of definiteness and regardless of whether the sharply defined edge is the leading or trailing edge of the respective film frame.
The just described proposal also failed to gain widespread acceptance in the industry because the size of film frames and/or the width of frame lines often varies from film to film, especially if the examination of a film which was exposed in a first camera is followed by examination of a film which was exposed in another camera. This will be readily appreciated by bearing in mind that the film windows of cameras (even of cameras produced by the same manufacturer) are not identical. In many instances, the neighboring frames of a film overlap each other. Furthermore, it happens quite frequently that the boundaries between neighboring frames include so-called film veils which also contribute to inaccuracies in connection with ascertainment of the positions of successive film frames. operate independently of each other. Thus, if one of the photosensitive devices detects a sharply defined edge between a film frame and the neighboring frame line, such edge will be used as a reference point for determination of the other edge regardless of whether or not the other edge has been detected with the same degree of definiteness and regardless of whether the sharply defined edge is the leading or trailing edge of the respective film frame.
The just described proposal also failed to gain widespread acceptance in the industry because the size of film frames and/or the width of frame lines often varies from film to film, especially if the examination of a film which was exposed in a first camera is followed by examination of a film which was exposed in another camera. This will be readily appreciated by bearing in mind that the film windows of cameras (even of cameras produced by the same manufacturer) are not identical. In many instances, the neighboring frames of a film overlap each other. Furthermore, it happens quite frequently that the boundaries between neighboring frames include so-called film veils which also contribute to inaccuracies in connection with ascertainment of the positions of successive film frames.